The invention concerns an automatic focusing device for slide projectors in which at least one photo-electric receiver of the secondary projection device which determines the position of the slide generates, from a beam of light impinging upon the receiver, the error voltage controlling the focusing of the slide. The error voltage is fed into an electronic amplifier circuit of the focusing device, which contains manual facilities for focus adjustment of the slide.
A device of this type is intended to allow readjustment of the image sharpness for the projected slide which, due to its condition (glass enclosure, extreme "popping") can no longer be focused adequately by the automatic focusing device, or whose projected image is out of focus when subjectively assessed.
Auto focus slide projectors of the prior art, for example, disclosed in DE-AS 1,472,300 or DE-PS 1,911,957, employ infrared control rays, the reflection of which is utilized at the slide emulsion carrier for control purposes. Here, the slide or the control arrangement are readjusted in such a way that the reflected radiation passing a slit diaphragm enters adjacently between two photo-electric receivers. This boundary position corresponds to the mechanically predetermined zero position of the control arrangement. For projection equipment of this type which is not permanently installed, the boundary position requires that when the first slide is introduced, the projection lens be set as precisely as possible to maximum sharpness; the use of a test slide to accomplish this is preferable.
With such an arrangement it is, of course, possible to adjust the sharpness using remote control, whereby the aperture or the projection lens is readjusted with a separate drive. However, the extra motor required for this function means considerable expenditure.
In DE-GM 1,979,885, there is disclosed a slide projector having an automatic focusing device and an additional focusing device operating manually via remote control, both served by the same positioner for sharpness adjustment. The two focusing devices are equipped with positioner control elements operating independently from one another. Simultaneous operation of these control elements is prevented by means of a changeover switch.
In order to effect a correction of the basic or initial sharpness position, the auto focus device described in DE-OS 2,333,128 contains facilities which add an adjustable voltage to the error voltage determined by the photo-electric receiver and thus cause the auto focus device to be disabled in a position allowing readjustment.
It is a disadvantage of the device described in the two publications mentioned immediately above that the readjusted position is maintained. As a consequence, only slides of identical structure can be used thereafter. Renewed refocusing is required each time slides of differing structure are to be used.